The military version of the AR15 rifle, including but not limited to the M16A1, M16A2, M16A3, M16A4, M4, M4A1, and the proposed M4A1-Plus are capable of operator-selected semi-automatic and full automatic fire. The fire control group is based on U.S. Pat. No. 3,045,555 to E. M. Stoner and makes use of a separate sear which allows proper operation of the firearm when automatic fire is selected. This additional sear is known as the “automatic sear,” and often referred to as a “trip sear.” According to U.S. Pat. No. 3,045,555, the automatic sear is spring biased and located within the lower receiver of the firearm and to the rear of the trigger components which include a hammer, trigger component, and an intermediate sear. The automatic sear functions to catch and release the hammer with the appropriate timing when automatic fire is selected. The spring bias for the automatic sear in this prior art arrangement is provided by a spring element operating between a surface of the automatic sear and a fire control selector for the fire control group. This fire control selector is also mounted on the lower receiver and used to select between semi-automatic operation, full automatic operation, and a safe condition in which the trigger mechanism is immobilized.
The modular trigger for the AR15 platform rifle shown in U.S. Pat. No. 7,293,385 to McCormick, has proven to be of utility by allowing easier removal and replacement of the trigger mechanism. It is possible to use a modular trigger such as that disclosed in U.S. Pat. No. 7,293,385 which will function with the automatic sear shown in U.S. Pat. No. 3,045,555. However, whether the trigger components are contained in a module such as that shown in U.S. Pat. No. 7,293,385 or otherwise, the automatic sear must be removed in order to remove the trigger module. The automatic sear must then be replaced after re-installing the trigger components, either via a modular trigger arrangement or otherwise. Removal and replacement of the automatic sear is complicated by the spring bias provided between the automatic sear and the fire control selector. This requirement of removing the automatic sear from the receiver to change the trigger mechanism prevents the installation/replacement of the prior art trigger module from being easily accomplished by a field level maintainer. Placement of the automatic sear as shown in U.S. Pat. No. 3,045,555 also constrains the design of enhanced function in the trigger assembly.